1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus.
2. Description of the Related Art
An image forming apparatus such as a copier, a printer, a facsimile machine generally includes a fixing device that fixes a toner image on a recording medium (hereinafter, “sheet”) such as a transfer sheet or an overhead projector (OHP) sheet by heat and pressure. Japanese Patent Applications Laid-open Nos. H11-167296, 2005-301043, and 2005-338761 disclose such conventional fixing devices as a heat-roller fixing device, a belt fixing device, and a pressure-belt fixing device. The heat-roller fixing device includes a fixing roller (heat roller) that comes into pressure-contact with a pressure roller. The belt fixing device uses an endless fixing belt as a fixing member. The pressure-belt fixing device includes an endless pressure belt with which a fixing roller comes into pressure-contact.
In these conventional fixing devices, a fixing roller or a pressure roller may include an elastic layer. Alternatively, a pressing member (also referred to as a pushing member or a pressure member) that presses a fixing belt or a pressure belt to form a nip may include an elastic layer or may be formed of an elastic material (hereinafter, elastic layer and elastic material are collectively referred to as “elastic member”).
Generally, the term “nip width” means a length of a nip portion in a sheet conveying direction. In the following explanation, however, a length of a nip portion in a sheet conveying direction is referred to as nip length, and a width of a nip portion in a direction perpendicular to the sheet conveying direction (roller axial direction) is referred to as nip width.
A member that directly contacts a recording medium on a fixing side (heating side) such as a fixing roller or a fixing belt is referred to as fixing member. A member that directly contacts a recording medium on a pressure side such as a pressure roller or a pressure belt is referred to as pressure member. A member that presses a pressure belt to form a nip is referred to as pressing member. A member that presses a fixing belt to form a nip on the fixing side is also referred to as pressing member.
In a fixing device, it is necessary to secure a nip long enough to obtain required fixing properties. Besides, it is desirable that nip length be uniform in the axial direction (i.e., nip width be uniform in the sheet conveying direction). When nip length is not uniform in the axial direction, even if, for example, a sufficient nip length can be obtained near the center of a sheet in a width direction, a nip length can be short at edges of the sheet in the width direction. In this case, fixing properties decrease at edges in the sheet-width direction.
In the conventional fixing devices, the elastic member is deformed due to pressure contact, which causes nip width to vary, resulting in a nonuniform nip shape (nip length is nonuniform in the axial direction). As a result, fixing properties may decrease. Thus, there is a need of a technology for preventing deformation of an elastic member at a nip portion.